Did that Dolby B noise reduction thing ever really work?

It is commonly thought that the Dolby button on a cassette deck does nothing except make the sound dull. It's supposed to make the sound less noisy. So why doesn't it work properly?

By
David Mellor

Friday November 25, 2005

The odd thing is that the cassette format still hasn't gone away. It's a fairly minor thing in the USA and Western Europe, but in South America, Africa, Asia and other rather significant slices of the world map it is still going very strong indeed.

But there is one thing about the cassette format that is commonly misunderstood, even by people who are otherwise knowledgeable about sound engineering...

That Dolby button actually did work! And still does.

There is a common feeling about Dolby, as applied to cassettes, that it isn't any use. Best to leave it switched out or the sound will be incredibly dull.

So Dolby = dull? Actually this is a million miles from the truth, and the intention of the system.

Without Dolby noise reduction, the cassette format is very noisy. You would be lucky to get a signal to noise ratio of 45 dB, which means that the background tape hiss is very much in the foreground whenever the signal is not absolutely full strength. And between tracks it's like sitting on the beach listening to the roar of the sea.

The way Dolby B works is to boost high-frequency, low-level signals during the record process, so they are at a higher magnetic level on the tape. Then the inverse of this process is applied on playback, lowering the tape noise in the process.

It really does work well. But to work well, it relies on the cassette deck being set up correctly for the type of tape in use, and for it to be clean and working in tip top condition.

Now this is where we find the problems. Suppose for instance that the level on playback is less than the level on record. Now the Dolby B decoder will lower the levels of some signals that should not have been lowered. Since it operates a high frequencies, the result will be a sound that is duller than it should have been.

This can happen if the machine is not correctly aligned for the type of tape in use. It will also happen if the head is dirty, and this in itself dulls the high frequencies so now they are doubly dulled.

And whoever heard of anyone cleaning the heads on their cassette deck? It should be done daily, but very few people even clean the heads at all.

So although the purpose of the Dolby B noise reduction system is to make playback sound as close as possible to the original source, and the system needs to be switched in on both record and playback, in practice it just makes the sound dull. Or at least it seems to.

So there are three morals to this tale...

If you would like to call yourself a sound engineer, you should take the trouble to understand this.

On a cassette deck, either use the brand and type of tape that is recommended, or align the machine for the type of tape in use.